1. Field of the Invention
The present invention relates generally to a temperature compensation circuit for a Hall effect element and, more particularly, to a temperature compensation circuit which prevents temperature compensation resistors from having a voltage potential across them which varies when the output of a Hall element changes.
2. Description of the Prior Art
Hall effect elements are well known to those skilled in the art. For example, Hall effect elements have been used for many years in devices which sense the movement of magnetic material through a detection zone. Geartooth sensors and vane sensors have been used in automobiles to monitor the position of rotating objects. Geartooth sensors have been used to monitor the rotation of crank shafts and cam shafts in conjunction with an automobile engine and, in addition, certain types of geartooth sensors have been used in conjunction with automatic braking systems.
A problem that exists in the use and application of Hall effect elements biased with a constant voltage is that the Hall output voltage varies over temperature primarily as a function of the temperature coefficient of resistance (TCR) of the material in which the Hall element is formed. For example, the output from a typical Hall effect element formed in the N epitaxy, for a constant magnetic field imposed on it, at 25 degrees centigrade is approximately double its output at 150 degrees centigrade. For uses in applications where changes in temperature are expected, some means must be provided to compensate for this temperature induced change in the Hall output voltage. Known temperature compensation circuits typically use temperature sensitive resistors as part of an amplification circuit to compensate for the change in Hall output voltage as a function of temperature. In certain known temperature compensation circuits, epitaxial resistors are used as part of the feedback circuit of an amplifier. Since epitaxial resistors exhibit a resistance at 150 degrees centigrade which is approximately double their resistance at 25 degrees centigrade, the inverse relationship between the resistance change in an epitaxial resistor and the voltage change in a Hall effect element formed in the same epitaxy can be used as a canceling compensation method.
U.S. Pat. No. 4,760,285, which issued to Nelson on Jul. 26, 1988, discloses a Hall effect device with epitaxial layer resistive means for providing temperature independent sensitivity. In this linear Hall effect integrated circuit, the output signal of the Hall element is amplified by a circuit whose gain is determined by a resistor that is partially formed in the same epitaxial layer as the Hall element. A first amplifier stage is configured as a voltage to current converter and is connected through a current mirror to a second amplifier stage that is configured as a current to voltage converter.
U.S. Pat. No. 4,734,594, which issued to Nelson on Mar. 29, 1988, describes a canceling compensation method for a sensor with temperature dependent sensitivity. The Hall effect device described in this patent has null (i.e. zero magnetic flux density) offset voltage compensation. The output terminals of the Hall effect element, which are formed in an epitaxial layer, are connected to a differential current source. The sum of the first and second currents produced by the source is determined by a resistor formed in the epitaxial layer in which the Hall effect element is formed. It is powered by the same electrical source as the Hall effect element so as to produce a current which tracks the current through the Hall effect element as a function of both temperature and the electrical source. The current through the resistor is split by a pair of trimmable temperature insensitive resistors and supplied to a pair of cross-coupled current mirrors which supply the currents to the output terminals of the Hall effect element.
When epitaxial resistors are used in the signal path to compensate for the output voltage changes in a Hall effect element as a function of temperature, an additional problem is introduced into the Hall circuit. Since epitaxial resistors exhibit a change in their resistance as a function of the voltage potential across them, their use in the feedback circuit of an amplifier can introduce errors in the resulting output of the Hall circuit. Although the effect on the resistance of an epitaxial resistor as a function of the voltage potential across it is relatively small, this variability in the resistance can deleteriously impact a Hall effect amplification circuit with an extremely high linearity requirement.
In order to avoid this problem that is caused by the change of the resistance in an epitaxial resistor as a function of the applied voltage potential, it would be significantly beneficial if a temperature compensation and amplification circuit could be provided which utilizes the epitaxial resistors in a configuration where the applied voltage potential is constant.